The interpretation given is not correct. When the flask containing air is put on the scales, the weight of flask plus air contained in the flask is being found, less the weight of the volume of air which the flask can contain (according to Archimedes' principle), i.e. the weight of the flask alone is found (without air). Then, when the air has been evacuated from the flask, the flask is weighed and this time the weight of the flask alone (without air) is found, less the weight of air which the flask can contain. The difference between these two weighings gives the weight of air which the flask contains.